Refrigerated vehicles have long been employed in a wide variety of applications including storage and transportation of perishable commodities, particularly produce such as fruit and vegetables, as well as other perishable foods, including processed and frozen or chilled products such as ice cream or the like. The refrigerated vehicles contemplated by the present invention include, for example, trucks, truck trailers, and refrigerated vans.
In such vehicles, it has been common to drive the refrigeration compressor by way of direct drive arrangement with the vehicle engine. While operating at higher speeds as when on the highway, for example, the compressor speed is normally sufficient to provide adequate capacity to the refrigeration system. However, when idling or operating at low speed city delivery conditions, the engine speed, and thus the compressor speed, is sufficiently reduced so as to provide insufficient capacity for the system. In addition, the combination of selectable box temperatures, wide ranging ambient temperatures, and wide engine speed variations, often produce a mismatch between the desired compressor capacity and the actual compressor capacity.
One approach to solving this problem is that of substituting a direct drive compressor with an electrically driven variable speed compressor which, in turn, is operatively connected to an engine driven generator. Although effective, this approach is expensive since the cost of the generator is relatively high. Further, such an approach requires a high electrical demand, and the ac current must be converted to dc current and then back to ac current. There is therefore a substantial cost penalty using such an approach.
Other problems associated with a direct drive compressor during pulldown include those of overloading the compressor especially during periods in which the box is hot. That is, at the start of a pull down condition when the box is 100° F., for example, the direct drive compressor can easily be overloaded and result in clutch failure or otherwise be caused to shut down unless the load is limited. This is normally accomplished by way of a compressor pressure regulator, which is a throttling valve to reduce the flow of refrigerant from the evaporator to the compressor Thus, under the typical pull down operating cycle, during the initial stages when the box is hot, it is necessary to limit the suction pressure and the compressor pressure regulator needs to be in operation. During the later stages of pull down, when the temperature in the box has been reduced to a temperature such as, for example, −20° F., then the density of the suction gas is low, resulting in the direct drive compressor having insufficient capacity to meet the demands.